In connection with the search for petroleum and natural gas, seismic surveying has been performed for many years. Originally, dynamite or TNT was utilized by exploding it on the surface or in a bore hole to create compressional seismic waves that are reflected or refracted by geological structures within the earth. It is possible, by the use of geophones located on the surface of the earth, to receive and respond to these seismic waves. Such waves are converted into electrical signals which are recorded and interpreted to enable geologists to plot in three dimensions the underground structural configuration.
More recently, either vibrators or hydraulic or pneumatically actuated devices for impacting a base plate located in intimate contact with the ground have been utilized to generate seismic waves. Both compressional and shear waves may be generated. It has been determined that, geologically, an interpretation can be made of shear waves to provide additional information beyond that which can be obtained from compression waves.
In order to produce shear waves, apparatus similar to that shown in U.S. Pat. No. 4,712,641, issued Dec. 15, 1987, to Stephen V. Chelminski are frequently employed. The seismic source or generator disclosed therein utilizes an air gun to produce a powerful shock through a base plate mounted in intimate contact with the earth. The airgun is pivoted on the base plate and the tilting thereof is utilized to direct such shock at an angle relative to the surface vertical and consequently to create horizontally polarized shear waves.
Another example of mobile apparatus that includes a seismic generator capable of producing the desired seismic waves is illustrated in U.S. Pat. No. 4,660,674, issued Apr. 28, 1987 to Tom P. Airhart. In that patent a heavy mass is pivotally mounted on a frame carried by the vehicle. The mass is propelled at high velocity into a base plate that is located on the surface of the ground. The impact force is translated into the desired form of seismic waves.
Among the problems involved in such apparatus has been the difficulty in determining the angle at which the seismic source or generator should be positioned relative to the surface of the earth to produce the optimum shear waves while simultaneously producing the desired compressional waves. Manifestly, and despite the efforts to prevent the base plate from moving horizontally along the surface of the ground, such horizontal movement or translation will occur at some relative angular position of the generator.
Many factors contribute to the difficulty of determining the optimum angle. For example, the amount of energy being imparted, the configuration of the base plate and, particularly, the configuration of the portion of the base plate in intimate contact with the soil. The major problem lies in the varying soil conditions that are encountered while conducting the geological survey of even a relatively small area.